From a5eaa164a77182ef7178f31d3db40c179e36e556 Mon Sep 17 00:00:00 2001
From: Marek Kaput <marek.kaput@swmansion.com>
Date: Fri, 20 Feb 2026 10:25:42 +0100
Subject: [PATCH] fix: preserve leading nil in stab clause body

A -> clause body can begin with a semicolon token after optional newlines, and that semicolon represents an implicit leading nil expression in the clause rhs.

The previous detection logic did not recognize the `-> \n;expr` shape reliably, so the resulting AST dropped that leading nil and diverged from Code.string_to_quoted/2 output in with ... else clauses.

This change detects the leading semicolon by peeking past end-of-line trivia before consuming clause eoe tokens, preserving the expected rhs shape.

A regression assertion was added to `property test regression cases` for `with x <- 1 do :ok else _ -> \n;a end` to prevent this AST mismatch from returning.

fix #113
---
 lib/spitfire.ex        | 25 +++----------------------
 test/spitfire_test.exs |  4 ++++
 2 files changed, 7 insertions(+), 22 deletions(-)

diff --git a/lib/spitfire.ex b/lib/spitfire.ex
index be54d5c..7d8688f 100644
--- a/lib/spitfire.ex
+++ b/lib/spitfire.ex
@@ -1195,28 +1195,9 @@ defmodule Spitfire do
               nl -> [newlines: nl]
             end
 
-          # Check if we have a semicolon right after -> (possibly after eol)
-          # Check if there's a leading semicolon right after ->
-          # Handles both ";expr" and "\n;expr" patterns
-          has_leading_semicolon =
-            case peek_token(parser) do
-              :";" ->
-                true
-
-              :eol ->
-                # Peek at the next token after eol without consuming
-                # We need to manually check the token sequence
-                with {:eol, _} <- parser.current_token,
-                     # Look at the tokens list to find what comes after eol
-                     [{:";", _} | _] <- parser.tokens do
-                  true
-                else
-                  _ -> false
-                end
-
-              _ ->
-                false
-            end
+          # A semicolon immediately after `->` (with optional newlines in between)
+          # starts the clause body with an implicit nil expression.
+          has_leading_semicolon = peek_token_skip_eol(parser) == :";"
 
           parser = eat_eoe_at(parser, 1)
 
diff --git a/test/spitfire_test.exs b/test/spitfire_test.exs
index 1dff579..9b0109f 100644
--- a/test/spitfire_test.exs
+++ b/test/spitfire_test.exs
@@ -2301,6 +2301,10 @@ defmodule SpitfireTest do
       assert Spitfire.parse("%e.(){}") == s2q("%e.(){}")
       assert Spitfire.parse("%e.(1){}") == s2q("%e.(1){}")
       assert Spitfire.parse("%e.(a, b){}") == s2q("%e.(a, b){}")
+
+      # with/else stab body with leading semicolon after newline
+      assert Spitfire.parse("with x <- 1 do :ok else _ -> \n;a end") ==
+               s2q("with x <- 1 do :ok else _ -> \n;a end")
     end
   end